On Sun, 12 Sep 2010 09:14:34 +0100, Norman Billingham wrote:

I have a pair of ATC SCA50 active speakers. They make a great sound but
the amplifiers run all the time, even when there's no signal to amplify.
Since they run in class A there is a significant current draw 24/7. As
they are at opposite sides of the room, turning them off involves two
switches and doesn't get done unless we're going away.

My previous Bowers Active 1s had an audio signal sensor in each speaker
so they went onto standby mode if there was no audio and "woke up" when
audio was present. Given the cost of running the ATCs all the time I'd
like to do the same with them.

There obviously needs to be a "switch off" delay so that the speakers
don't power down in quiet passages.

Do any of you experts know if something like the Velleman 4700 would do
the job? Or of any other circuit? I'm perfectly happy assembling
something but I'm no circuit designer.

I had a similar problem and solved it using cheap radio controlled
switches, Maplin part number N79FN. These are currently five pounds each.
If you have a pair and set both to the same channel, you could turn both
speakers off and on at the same time.

This is not an automatic solution, but it should be both cheap and
effective.